This invention relates to optical pulse propagation.
Group velocity dispersion (GVD) is the main phenomenon restricting the maximum useable bit rate of optical fibre transmission systems. When a short pulse propagates down the fibre, different spectral components of the pulse travel with different velocities (due to GVD) which results in pulse broadening.
Considering a spectral region in which xe2x80x9cbluexe2x80x9d (shorter wavelength) components propagate faster then xe2x80x9credxe2x80x9d (longer wavelength) ones (and such a situation occurs for wavelengths longer than 1300 nm in conventional non-dispersion-shifted single mode fibres) then the leading edge of the pulse contains the shorter-wavelength spectral components while longer ones are found on the tailing edge of the pulse. In other words, the instantaneous frequency varies across the pulse and the pulse becomes xe2x80x9cchirpedxe2x80x9d.
Pulse broadening xcex94xcfx84 can be expressed in the form
xcex94xcfx84=Dzxcex4xcex,
where D is the chromatic dispersion in ps/nmxc2x7km, z is the length of the fibre and xcex4xcex is the spectral width of the pulse. It is this pulse broadening effect which limits the maximum pulse repetition frequency in a transmission system.
One acknowledged way of minimizing or reducing the pulse broadening is the use of dispersion-shifted fibres with parameter D close to zero. Another way to reach high bit-rates is to compensate for group velocity dispersion (instead of attempting to minimize it) using the nonlinear properties of the glass from which the fibre is made (usually doped silica).
It is known that the refractive index of silica glass can be expressed as a combination of linear and nonlinear components. The latter can be written in the form
nn1=n2I,
where n2 is the so-called nonlinear refractive index (which in silica glass is equal to 2.6xc2x710xe2x88x9216cm2/W) and I is the intensity of the light. The nonlinear part of the refractive index causes a phase change
xcfx86n1=kn2Iz=2xcfx84n2Iz/xcex,
(where xcex is the wavelength) and if the light intensity I depends on time (as found in a pulse of light) then the refractive index nonlinearity results in a variation of the instantaneous frequency across the pulse. Indeed, the pulse phase can be written in the form
xcfx86=xcfx890txe2x88x92xcfx861xe2x88x92xcfx86n1=xcfx890txe2x88x92knzxe2x88x92kn2I(t)z,
where xcfx890 is the pulse central frequency and xcfx861 is a linear phase shift. The first derivative of the pulse phase is the pulse instantaneous frequency
dxcfx86/dt=xcfx89=xcfx890xe2x88x92kn2z dI(t)/dt.
Thus the refractive index nonlinearity results in a pulse chirp with opposite sign to the dispersion-induced chirp (assuming fibre dispersion in the region longer than 1300 nm to be positive). A physical interpretation is that the fibre nonlinearity causes the red components of the pulse spectrum to travel faster than the blue ones and this effect can be used to compensate dispersion-induced pulse broadening.
It is clear that in order to cancel dispersion broadening of the pulse using refractive index non-linearity one needs a certain pulse intensity for a given pulse width xcfx84c and dispersion. Such pulses with dispersion broadening balanced by nonlinear compression are called solitons. Their intensity corresponds to the intensity of the so-called fundamental soliton Is and can be written in the form
IS=0.322xcex3D/(4xcfx802xcfx842c)
Thus the lower the dispersion and the broader the pulse the less intensity one needs to compensate for dispersion-induced broadening.
Solitons have a number of interesting properties, but the most important (for practical applications) soliton properties are listed below. These will be referred to later as properties #1 to #7.
1. A soliton is a bandwidth-limited pulse with time-bandwidth product xcfx84xcex94xcexd=0.315, where xcex94xcexd=cxcex4xcex/xcex2 is the soliton""s spectral bandwidth.
2. The soliton""s phase is constant across the pulse.
3. The soliton""s temporal shape is sech2t (where t is time).
4. The soliton""s intensity and pulse width are related to each other, namely
Pxcfx842=Constxc2x7D.
(typically P=10 mW for xcfx84=5 ps and D=1 ps/nmxc2x7km)
5. After some distance of propagation a non-sech2t pulse evolves into a soliton (sech2t) pulse and a non-soliton component.
6. A soliton accompanied by spurious radiation transforms into another soliton with modified parameters (central frequency, intensity, pulse width) and a nonsoliton component.
7. Two solitons closely situated in time interact with each other through overlapping optical fields. To avoid soliton interactions the separation time between solitons should exceed five times their pulsewidths.
When a soliton propagates down a fibre with loss then its intensity becomes less and, in accordance with soliton property #4, it becomes broader. When the soliton becomes broader it begins to interact with adjacent pulses which is not acceptable for transmission systems.
Another serious problem is associated with soliton property #5. During propagation in a lossy fibre the soliton remains an essentially nonlinear pulse for some distance which results in narrowing of its spectral bandwidth, but after that distance its intensity is insufficient for the compressive effect of the non-linear refractive index to adequately balance the fibre dispersion and the soliton experiences only temporal broadening without any significant changes in spectral bandwidth. This means that the original bandwidth-limited pulse becomes a chirped one. The longer the distance the soliton propagates the more it differs from the ideal sech2t-shape and therefore the bigger the fraction of non-soliton component in the propagating pulse. After amplification this non-soliton component is shed by the soliton according to soliton property #5. Physically the non-soliton component is a dispersive pulse which may propagate faster or slower than the soliton and can interact with the main pulse changing its parameters and even destroying it. The strength of this nonlinear coupling depends on the intensity of the non-soliton component and hence the more the soliton shape differs from the xe2x80x9cideal solitonxe2x80x9d the stronger the non-soliton component affect the propagation of the main pulse.
The situation becomes much worse when a soliton propagates in a transmission system which normally comprises a chain of fibre links and optical amplifiers. At each stage the soliton emits the non-soliton component, and after several amplification stages the level of non-soliton component becomes so high that nonlinear coupling between the two fields causes the soliton to break-up.
Thus interaction between soliton pulses and the accompanying non-soliton component results in soliton instability. One previously proposed way of reducing this effect is to keep the amplifier spacing much shorter than the soliton dispersion length zd, which is approximately equal to xcfx842/D. Doing so reduces the amount of the radiated non-soliton component at the expense of shortening the required amplifier spacing. The latter is expensive and is thus commercially undesirable.
For example, in standard telecom fibres with group velocity dispersion around 17 ps/nmxc2x7km the dispersion length is of the order of 0.5 km for 5 ps pulses and 40 km for 50 ps pulses and normally the amplifier spacing should be less than this distance. An improvement is obtained in dispersion-shifted fibre with typical dispersion 1 ps/nmxc2x7km, in which case the dispersion length and hence the appropriate amplifier spacing can be as long as 7 km for 5 ps pulses and 700 km for 50 ps pulses. Thus, practically speaking, the effect of soliton instability not only imposes a limitation on the pulsewidth and amplifier spacing but also dictates the use of dispersion-shifted fibres in soliton transmission systems.
There is another serious problem associated with soliton property #6. At each amplifier some small amount of background amplified spontaneous emission is added to the soliton. This noise causes random variations of the soliton parameters, including the soliton central frequency which translates into uncertainty in soliton arrival time, causing timing jitter.
In summary, nonlinear coupling between the soliton and the non-soliton component is a key parameter affecting soliton stability, and the best way to maintain stable soliton propagation in a system with reasonably long amplifier spacing is to reduce the amount of non-soliton component. For this reason a number of practical soliton transmission systems have proposed the use of so-called soliton transmission control techniques.
There are two main strategies in soliton transmission control. The prime target of the first one is to provide higher transmission loss for the non-soliton component than for the soliton which thus reduces the non-soliton component. Several methods have been proposed to minimise or reduce the non-soliton component.
In the first method (see publication reference 1 below, denoted as [1]) an amplitude modulator was timed to pass the solitons at the peak of its transmission. The pulses that were positioned off their assigned time slot were retimed. The main disadvantage of this scheme is the requirement to incorporate the extra modulator periodically along the length of the transmission system.
Another approach was demonstrated in [2] where the peak transmission frequencies of optical filters (so-called guiding filters) were gradually translated at each filter station along the fibre link. This makes the link substantially opaque to noise and transparent for solitons. The sliding frequency technique requires however a very precise set of optical filters which must be placed along the optical fibre link in the correct order.
A third previously proposed technique is the use of nonlinear gain (or loss). The main idea is to introduce additional losses for linear radiation relative to that experienced by the solitons using both filtering and a saturable absorber. If an excess linear gain at a fibre amplifier/saturable absorber combination is high enough to compensate the soliton loss experienced in the preceding fibre section, but not sufficient for the compensation of linear radiation loss then one can expect stable soliton propagation with a suppressed low-level non-soliton component. The main problem associated with this method is the finite recovery time of the saturable absorber.
Thus there is a need for an improved technique for stabilising optical soliton propagation along an optical fibre communication link. Such a technique could potentially allow higher bit rates and/or greater amplifier separations to be used.
This invention provides optical communication apparatus comprising:
a dispersive optical fibre link;
an optical transmitter for launching optical pulses into the optical fibre at a pulse intensity sufficient to provide non-linear dispersion compensation during propagation through a first length of the fibre;
a series of amplifier units spaced along the fibre by a distance greater than the first length of the fibre, each amplifier unit comprising a counter-chirping device for substantially compensating for the dispersion of a second portion of the fibre, being the difference between the first length and the amplifier spacing, and an amplifier for launching amplified optical pulses into the optical fibre at a pulse intensity sufficient to provide non-linear dispersion compensation during propagation through the first length of the fibre.
The invention provides a new hybrid technique for pulse propagation along an optical fibre. Initially, in a first portion of the fibre, the pulses launched into the fibre are sufficiently intense to provide non-linear dispersion compensation of the fibre dispersion. When in a second portion of the fibre, through fibre losses and pulse broadening, the pulse intensity is no longer sufficient to provide this effect, a counter-chirping device is provided to compensate for the dispersion of the second portion.
This hybrid linear/non-linear technique can advantageously increase the possible amplifier spacing in a transmission link.
The counter-chirping device can be attached anywhere along the second portion, but is preferably a part of an amplifier package at the output of the second portion.
It will be appreciated that the launching step could be carried by launching the pulses from an optical transmitter, another fibre, an amplifier and so on.
Embodiments of the invention addresses the problems of obtaining stable soliton propagation by controlling the amount of the non-soliton component. This technique will be referred to as chirped bandwidth-limited amplification (CBLA).
As mentioned above, when a soliton propagates in a fibre with loss its intensity becomes less and thus it becomes broader and transforms from a bandwidth-limited pulse to a chirped one. After amplification, a chirped nonlinear pulse tends to split into a fundamental soliton and a nonsoliton component and therefore after several amplification stages the fraction of non-soliton component becomes unacceptably high and causes the soliton to break-up. (It should be pointed out here that injecting a higher intensity pulse into the fibre in order to compensate for fibre losses and make use of the multisoliton compression effect to compensate for the soliton broadening is inefficient owing to the generation of an even hither level of the non-soliton component which again quickly destroys the soliton).
However if a counter-chirp is deliberately imposed over the soliton to compensate for the fibre dispersion-induced chirp then the soliton emits much less linear radiation and can propagate much longer distances without breaking up. Thus in embodiments of the invention, the use of chirped bandwidth-limited amplification results in stable soliton propagation.
The invention also provides a method of optical communication comprising:
launching optical pulses into a dispersive optical fibre link at a pulse intensity sufficient to provide non-linear dispersion compensation during propagation through a first length of the fibre; and
providing a series of amplifier units spaced along the fibre by a distance greater than the first length of the fibre, each amplifier unit comprising a counter-chirping device for substantially compensating for the dispersion of a second portion of the fibre, being the difference between the first length and the amplifier spacing, and an amplifier for launching amplified optical pulses into the optical fibre at a pulse intensity sufficient to provide non-linear dispersion compensation during propagation through the first length of the fibre.
Other aspects and preferred features of the invention are defined in the appended claims. It will be appreciated that preferred features defined with respect to one aspect of the invention are applicable to other aspects of the invention.